Neptune's rate of rotation has been accurately determined, something extremely difficult to do with the solar system's gas giant planets, U.S. astronomers say.

By tracking atmospheric features on Neptune, University of Arizona planetary scientist Erich Karkoschka has found a day on the gas-covered planet lasts exactly 15 hours, 57 minutes and 59 seconds.

His result is one of the largest improvements in determining the rotational period of a gas planet in almost 350 years since Italian astronomer Giovanni Cassini made the first observations of Jupiter's Red Spot, a UA release said Thursday.

"The rotational period of a planet is one of its fundamental properties," said Karkoschka. "Neptune has two features observable with the Hubble Space Telescope that seem to track the interior rotation of the planet. Nothing similar has been seen before on any of the four giant planets."

Unlike the rocky planets of the inner solar system -- Mercury, Venus, Earth and Mars -- which are solid balls spinning in a straightforward manner, the giant gas planets -- Jupiter, Saturn, Uranus and Neptune -- rotate more like giant blobs of liquid with a lot of sloshing, swirling and roiling which has made it difficult for astronomers to get an accurate grip on exactly how fast they spin.

"On Neptune, all you see is moving clouds and features in the planet's atmosphere. Some move faster, some move slower, some accelerate, but you really don't know what the rotational period is, if there even is some solid inner core that is rotating," Karkoschka said.

Using Hubble Space Telescope images of Neptune, Karkoschka was able to track two permanent features of the planet's atmosphere, known as the South Polar Feature and the South Polar Wave, to accurately determine the planet's rotation rate.

"The regularity suggests those features are connected to Neptune's interior in some way," Karkoschka said. "How they are connected is up to speculation."